Low temperature zn-al-cu casting alloy

ABSTRACT

ALLOYS HAVING LOW MELTING POINTS AND HIGH CREEP RESISTANCE ESPECIALLY ADAPTED FOR CASTING MOLDING TOOLS, E.G., STAMPING DIES CONSISTING ESSENTIALLY OF ALUMINUM, ZINC AND COPPER WHEREIN THE AMOUNT OF ALUMINUM IS FROM ABOUT 10 TO ABOUT 30% BY WEIGHT AND THE AMOUNT OF COPPER IS FROM ABOUT 10 TO ABOUT 20% BY WEIGHT, SAID PERCENTAGES BEING BASED ON THE WEIGHT OF THE ALLOY. THE ALLOY MAY ADDITIONALLY CONTAIN UP TO ABOUT 2% BY WEIGHT OF NICKEL AND/OR IRON.

United States Patent Otfice 3,671,227 Patented June 20, 1972 3,671,227 LOW TEMPERATURE Zn-Al-Cu CASTING ALLOY Herbert A. Jahnle, Havertown, Pa., assignor to The Budd Company, Philadelphia, Pa. No Drawing. Filed Sept. 30, 1969, Ser. No. 862,482 Int. Cl. C22c 17/00 US. Cl. 75-178 AC 1 Claim ABSTRACT OF THE DISCLOSURE Alloys having low melting points and high creep resistance especially adapted for casting molding tools, e.g., stamping dies consisting essentially of aluminum, zinc and copper wherein the amount of aluminum is from about to about 30% by weight and the amount of copper is from about 10 to about by weight, said percentages being based on the weight of the alloy. The alloy may additionally contain up to about 2% by weight of nickel and/or iron.

BACKGROUND OF INVENTION The invention relates to novel low melting alloys having high hardness and high strength and which can be precision cast into molding tools.

Molding and forming tools such as punches, stamping dies, etc., for working and forming sheet metal and the like have been in use for a long period of time. In the past many compositions have been suggested as base materials for these tools. Ferrous base alloys such as cast iron, cast steel and tool steels are normally used for high production such as that found in the automotive or appliance' fields. Zinc base alloys such as Kirksite and Formalloy are used for relatively low production or prototypes. Such zinc base alloys have a nominal composition of 4% aluminum, 4% copper, 0.1% magnesium and remainder zinc.

Cast iron which is widely used as a forming tool material in large production, 100,000 parts, has a moderate hardness, l90250 Brinell, and tensile yield strength, 35,- 000 to 55,000 p.s.i. The zinc base alloys mentioned above have a typical tensile strength of 35,000 p.s.i. and a typical Brinell hardness of 100. These zinc base alloys do not possess a yield strength, as normally found in most metals, because of their poor room temperature creep resistance. Cast iron does not creep at room temperature at 45,000 p.s.i. after it takes an initial set. Kirksite as an example will creep considerably .100 inch/inch in one hour at 45,000 p.s.i. The comparison is made at 45,000 psi. stress because it is known from production experience that cast iron is a very acceptable material for almost all parts while Kirksite is used for very short runs only usually less than 1,000 parts. Undoubtedly, there are parts requiring very small forming forces where Kirksite could be used for larger production.

Kirksite or Formalloy is used instead of cast iron for low production even though Kirksite metal is more expensive on a per pound basis. The reason for this is that Kirksite can be made into forming tools at a lower total cost than cast iron. Since the zinc base alloys are cast at a much lower temperature, 850 F., then cast irons, above 2200 F., the resultant finish tolerance can be controlled to a much greater degree. Only minor machining is required for the zinc alloys prior to use but cast iron tools must be machined extensively before use. The machining costs on complex shapes of forming tools are very high due to the skill of the operators and expensive equipment required.

SUMMARY OF THE INVENTION It has been found that the addition of certain amounts of copper to certain aluminum-zinc alloys functions to maintain or lower the low melting point of the alloy and increase its hardness and creep resistance properties under high compression loads Thus, the advantages of the alloys of the invention are two-fold. First, the ease and efliciency of methods for melting and precision casting tools from these alloys is enhanced due to their lower melting point. Since lower heat requirements accompany the formation of tools from the alloys of the invention, great savings are realized by the use' of melt casting systems which require lower heat outputs than those employed to cast the alloys heretofore utilized.

Second, since the creep resistance and hardness properties of the alloys of the invention are acceptable at high compression loads, molding tools constructed thereof find a wider variety of applications than those composed of presently used zinc base alloys.

The alloys of the present invention and the tools constructed thereof consist essentially of aluminum, zinc and copper wherein the amount of aluminum is from about 10 to about 30% by weight and the amount of copper is from about 5 to about 20% by weight, said percentages being based on the weight of the alloy. Optionally, up to about 2% by weight based on the weight of the alloy, of nickel and/or iron may be included in the alloy as hardening agents.

Molding tools such as punches, stamping dies, etc., may be constructed from the above-described alloys by conventional melt casting methods.

The series of alloys of this invention have an additional advantage in that they form metallurgical bonds with metals such as nickel, copper and iron. For this reason they can be used as back up for electroformed molds of the above-mentioned metals. Electroforrning shells of models provides a means of accurately copying a complex shape. The shell is then bonded to the alloys of this invention to provide intimate support and methods of mounting in tools, die sets, presses, etc. The cast back up provides metallic integrity necessary for thermal conductivity, etc. This system of metallic back up of electroformed shells permits the mold face presented to the moldable substance, such as plastics, ceramics, glass, etc. materials to be compatible with the moldable substance.

DETAILED DESCRIPTION OF THE INVENTION As mentioned hereinabove, the addition of copper to certain aluminum-zinc alloys serves to decrease or maintain the melting point (liquidus) thereof. The effect on the melting point of various aluminum-zinc alloys produced by varying the percentage of copper therein is shown in Table 1.

TABLE 1 Percent Percent Melting Percent zinc aluminum copper point F.

45,000 p.s.i. stress for /2 hour and compared with an alloy sample containing 20% aluminum and 80% zinc which had been subjected to the same stress conditions but for only one minute. The latter was found to have undergone a dimensional change or creep .236 inch per inch due to the stress conditions Whereas the dimensions of the copper containing alloy sample of the invention did not change.

A sample of Kirksite was also compression tested at a stress of 45,000 p.s.i. and it exhibited a dimensional change or creep of .031 inch per inch in twenty minutes.

The eifect of copper on the hardness of castings prepared from the alloys of the invention is evident from Table 2. Samples of the various alloys described therein were melt casted in a sand mold in the conventional manner.

TABLE 2 Percent Percent Brinell Percent zinc aluminum copper hardness TABLE 3 Temperature F. Rockwell B hardness As cast 69 A Rockwell B hardness of 84 is equivalent to a Brinell hardness of 1'60.

It is readily apparent that the presence of copper in the aluminum-zinc alloys increases the hardness of the as cast alloys and the quench hardened samples thereof.

The alloys of the invention consist essentially of from about 10 to about 30% of aluminum, about 10 to about 20% of copper, about 0 to about 2% nickel and/or iron and the remainder, zinc, said percentages being by weight based on the weight of the alloy. It is to be understood, however, that any of the additives, hardeners, conditioners, etc., commonly incorporated in alloys, which do not substantially affect the properties thereof may be added to the alloys of the invention.

The alloys in this investigation so far have been prepared by melting the proper amount of zinc at a furnace temperature of 800 to 900 F. After the zinc has completely melted, the commercially pure aluminum ingot is added and pushed beneath the zinc surface.

Wetting the aluminum completely with zinc appears to increase the melting rate. The furnace temperature is increased to 1000" F. until the aluminum is fairly well completely melted. The furnace temperature is raised to 1200 F. and copper, commercially pure ingot, is added. When the copper is completely dissolved the furnace temperature is lowered to 850 to 900 F., stirred, and held at this temperature until pouring. The aluminum and copper could be melted by leaving the furnace temperature at 900 F. but longer times would be required. The highest temperature used in melting is limited due to oxidation of the melt and volatilization of the zinc. Master alloys of aluminum-copper, copper-Zinc or aluminum-zinc or combination of all three with the necessary pure metal to arrive at the final desired composition could be used.

As stated above, the alloys of the invention are advantageous in that they may readily be melt casted at relatively low temperatures due to their depressed melting points. Generally, the molding tools of the invention are prepared by melt-casting the alloy in a sand mold having the desired configuration in accordance with methods well known in the art. Due to the lowered melting points of the alloys of the invention, however, the castings prepared therefrom may be poured from furnaces at temperatures considerably below 1000 F., preferably about 900 to 950 F.

Molding tools of any shape or configuration, preferably those employed in applications requiring high hardness and creep resistance under high compression loads such as sheet metal working operations, etc., may be constructed from the alloys of the invention. Such tools are well known in the art and take the form of punches, stamps, mold patterns, etc. Since the hardnesses of the alloys of the invention compare favorably with those of cast iron which is a commonly used molding tool composition, i.e., 190 to 250 (Brinell), they may be substituted for cast iron in most of the applications of the latter. If desired, in those applications involving unusually severe wear conditions, hardened steel inserts may be cast into the molding tools of the invention during forming.

Laminated or composite molding tools can be constructed using the alloys of this invention. Electroformed shells made of various metals such as nickel, copper, iron, etc., can be made from models using procedures known in the trade. Since the time and expense required to make heavy or thick electroformed shells are high it is desirable to build up the mold section by casting a metallic alloy against the electroform shell. The alloy cast should form a metallurgical bond with the metal comprising the electroform shell for good thermal conductivity, strength, etc.

The invention will be further illustrated by the following non-limiting examples.

Example 1 A flat bottom cup forming punch and draw ring were cast in a sand mold from a furnace having a temperature of 900 F. using an alloy composed of 15% copper, 20% aluminum and 65% zinc. The Brinell hardness after quenching the cast from 700 F. was 174 to 183.

The punch and draw ring were used to form one thousand sheet metal cups from carbon steel (automotive grade), cups of aluminum alloy 5052, and 20 cups from type 301 stainless steel. Measurements of the punch and draw ring dimensions before and after forming showed no wear.

Example 2 A punch similar to that of Example 1 was cast in a similar manner by pouring from a furnace having a temperature of 1000 F. an alloy composed of 10% aluminum, 15 copper and 75% zinc. The tool had a melting point of 910 F. The as cast Brinell hardness and the quenched (705 F.) Brinell hardness of the punch was 137. The punch was utilized in a sheet metal working operation and underwent no dimensional change.

Example 3 A punch similar to that of Example 2 was prepared in the same manner from an alloy composed of 30% aluminum, 15 copper and 55% zinc. The tool had a melting point of 890 R, an as cast Brinell hardness of 131 and a quenched (705 F.) Brinell hardness of 163.

The punch showed no dimensional changes when utilized in a conventional sheet metal working operation.

Example 4 A punch similar to that of Example 1 was prepared in the same manner from an alloy composed of 15% copper, 20% aluminum, 2% nickel and 63% zinc. The tool had a melting point of 810 R, an as cast Brinell hardness of 156 and a quenched (705 F.) Brinell hardness of 179.

The punch showed no evidence of creep when subjected to compression loads up to 45,000 p.s.i. for extended periods of time.

Example 5 A punch was prepared according to the method set forth in Example 1 from an alloy composed of 15% copper, 20% aluminum, 2% iron and 63% zinc. The alloy had a melting point of 810 F. The punch prepared thereform was found to have an as cast Brinell hardness of 156 and a quenched (705 F.) Brinell hardness of 197.

The punch was subjected to compression loads up to 45,000 p.s.i. with evidence of creep.

Example 6 A nickel electroform which was in the shape of a 4 inch diameter cup with a inch wall height was backed with a copper-20% aluminum-70% zinc alloy in the following manner. A 2 /2 inch stainless steel sheet metal ring was made 4 inches in diameter to increase the cavity depth to 2 /2 inches. The nickel electro form was pre-heated to approximately 750 F. on a hot plate and with a gas torch. The nickel surface was tinned with a 5% aluminum-zinc alloys by melting and scrubbing with wires of this alloy. After complete wetting of the nickel surface the electroform was placed in an oven at 600 F. and allowed to stabilize at this temperature. The 10% copper-20% aluminum-70% zinc alloy, held in the molten state at 1000 F. was then poured into the cavity. A thermal insulating block was placed on top of the cavity and the electroform removed from the furnace to develop directional solidification starting at the nickel-casting interface. This produces a pipe or void free casting at the nickel surface. The normal shrinkage upon solidification then occurs at the top of the casting and can be removed by machining if desired. This completed casting was then pressure tested at 20,000 p.s.i. with no dimensional changes.

What is claimed is:

1. An alloy having a low melting point and high creep resistance under high compression loads at room temperature consisting essentially of aluminum, zinc and copper wherein the amount of zinc is about by weight, the amount of aluminum is about 20% by weight and the amount of copper is about 15% by weight, said percentages being based on the weight of said alloy.

References Cited UNITED STATES PATENTS 1,350,893 8/1920 Tedesco 178 AC 1,540,006 6/1925 Hodson 75-178 Ac 2,982,677 5/1961 PC1261 14813 1,364,654 1/1921 Tedesco 75178 AC 1,506,772 9/1924 Pack 75-178 R 1,832,733 11/1931 Peirce et a1. 75-178 6 FOREIGN PATENTS 478,112 2/1953 Italy 75-178 AC L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner 

